The present invention relates to a method for manufacturing a motor yoke, a motor yoke manufactured with this manufacturing method, and a motor equipped with the yoke.
A typical motor includes a cylindrical yoke and a cylindrical stator core, which is accommodated in the yoke. The stator core is secured to the yoke by press fitting or shrink fitting the stator core in the yoke. The yoke generally has a cylindrical portion and a bottom portion, which is integrally formed with the cylindrical portion to close one end of the cylindrical portion. The axially middle part of the cylindrical portion forms a securing portion to which the stator core is secured. If the dimensional accuracy of the securing portion is low, the circularity of the inner diameter of the stator core is decreased when press fitting or shrink fitting the stator core in the securing portion. This causes the air gap between the stator core and a rotor surrounded by the stator core to become uneven, which increases cogging and decreases the efficiency of the motor. Therefore, the dimensional accuracy of the securing portion needs to be improved.
The yoke is generally formed by drawing of plate material. The circularity of the cylindrical portion of the yoke formed by drawing is approximately 0.1 [mm]. To achieve the above mentioned accuracy, a technique for machining a tube disclosed in Japanese Laid-Open Patent Publication No. 2003-225725 may be applied. The technique disclosed in the above publication permits a tube having a weld bead such as an electric welded tube to be plastically deformed with a high dimensional accuracy.
More specifically, a swaging machine disclosed in the above publication includes a columnar mandrel, which is selectively inserted into the electric welded tube, a cylindrical die, which is selectively fitted to the mandrel, a pressing die, which selectively approaches and separates from the electric welded tube from radially outside of the mandrel, and a clamp, which secures the electric welded tube. A step is formed on the outer circumferential surface of the mandrel. The mandrel also includes a narrow tapered portion, which is connected to the step, a parallel forming part, which is connected to the tapered portion, and an escape portion, which is connected to the parallel forming part. The diameter of the escape portion is slightly smaller than that of the parallel forming part. The die includes a cylindrical inner surface the diameter of which is the same as the outer diameter of the electric welded tube after being formed, such that the outer diameter of the electric welded tube can be made equal to a desired outer diameter. When the mandrel is fitted to the die, a cylindrical space, which has the width that is the same as the thickness of the electric welded tube, is formed between the cylindrical inner surface and the parallel forming part, and a cylindrical space, which has the width that is greater than the thickness of the electric welded tube, is formed between the cylindrical inner surface and the escape portion. A tapered introducing portion is formed at the opening end of the die to introduce the electric welded tube into the die. The pressing die approaches the electric welded tube that has been machined using the mandrel and the die from radially outside and crushes the weld bead of the electric welded tube.
When machining the electric welded tube, at first, the electric welded tube is secured to the clamp. The die is then moved along the axial direction until the distal end of the electric welded tube contacts the tapered introducing portion of the die. On the other hand, the mandrel that is inserted in the die is inserted into the electric welded tube until the distal end of the electric welded tube contacts the step of the mandrel. After that, the die is slightly moved forward such that the die approaches the electric welded tube. This draws the electric welded tube along the tapered introducing portion. In addition, the inner rim of the distal end of the electric welded tube is pressed against the tapered portion of the mandrel so that a tapered surface is formed at the distal end. After that, by further advancing the die, a swaging process is performed so that the distal end of the electric welded tube is gradually drawn. In the swaging process, the distal end of the electric welded tube is pressed by the cylindrical inner surface of the die and the parallel forming part of the mandrel. Accordingly, the distal end is formed to have the desired outer diameter and the inner diameter. After the swaging process, the pressing die approaches the electric welded tube as the die is retracted. Accordingly, the weld bead of the inner surface of the electric welded tube is pressed against the parallel forming part and crushed. As a result, the inner surface of the distal end of the electric welded tube is made flat and smooth.
However, the technique disclosed in the above publication is suitable for machining the axial ends of a cylindrical body, but is not suitable for machining the securing portion located at the axially middle part of the motor yoke. That is, when machining the motor yoke using the technique of the above publication, the yoke needs to be continuously machined from the opening end of the cylindrical portion of the yoke to a section close to the bottom portion of the yoke. Therefore, not only the axially middle part of the yoke that will serve the securing portion, but also other part is machined in the same manner as the securing portion. This causes a waste of machining.